Binary complexes of HCN with H2, HD, and D2 formed in helium nanodroplets

Rotationally resolved infrared spectra have been obtained for HCN–oH2, pH2–HCN, HCN–pD2, HD–HCN, and oD2–HCN binary complexes formed in liquid helium nanodroplets. The isolation provided by the droplets enables us to observe both ortho and para complexes, while in the previous gas phase infrared study only the more strongly bound J=1 complexes were observed. Of particular interest is the observation that the complexes correlating with J=0 and J=1 hydrogen bind to opposite ends of the HCN molecule. In an effort to understand this behavior we have carried out ab initio calculations at the CCSD(T) level, exploring the potential energy surface near the corresponding minima. The dramatic difference between the binding of ortho and para hydrogen to HCN can be at least partially understood by averaging the calculated surface over the appropriate rotational wave functions.